1. Field of the Invention
This invention relates to a method of producing clad metal, more particularly to a method of cladding the surface of a metal with a layer exhibiting corrosion resistance, resistance to hot corrosion, oxidation resistance, wear resistance and other superior characteristics.
2. Description of the Prior Art
Recent industrial and technological advances have been creating a need for materials that can be used in increasingly severe environments. The field of energy resource development is one example. Development is now being directed to recovery of fluids such as sour oil and sour gas, i.e. petroleum and natural gas containing large quantities of hydrogen sulfide and carbon dioxide. Tubular goods and linepipes made of low alloy steel are not suitable for this work since they are apt to corrode and crack. As a result, Ni-base alloy products such as Hastelloy C-276 and Inconel 625 (tradenames) are already being used. The high price of these metals is, however, a major problem. It has therefore been contemplated to use clad steel goods having one of these alloys only as a cladding, the required strength being provided by the metal substrate (low alloy steel, for example).
Various methods for producing clad steel products have been proposed, specifically for producing tubular goods such as seamless pipes or welded pipes and flat products as rolled plates. In all cases, however, the process is complicated and the yield is low. What is more, it has been found difficult to produce clad steels which use Hastelloy C-276 or Inconel 625 as the cladding material. This difficulty is even greater in the case of clad steel tubes and no practicable method has been developed heretofore. Studies carried out by the inventors show that this difficulty results from the fact that in the course of hot working the flow stress exhibited by these alloys is much greater than that exhibited by the metal substrate. Thus hot working and other conventional production process cannot be used since the two types of metal deform independently of each other, making it impossible to uniformly process the cladding and the metal substrate. This makes bonding of the two metals difficult.
Clad steels are also used in other applications. It is common, for example, to clad the sliding surfaces of valve spindles, the piston and cylinder walls of reciprocal pumps, and the inner surface of pipes for carrying slurries, so as to make them more resistant to wear. In these cases, a cladding of an alloy such as Stellite (tradename) is applied by overlaying or spraying. Further, pressure vessels and steel pipes used at high temperatures are provided by overlaying or spraying with a cladding of oxidation resistant material such as Ni--Cr alloy, Ni--Cr--Al--Y alloy or Co--Cr--Al--Y alloy. However, in all such cases it is the finished product that is provided with the cladding by overlaying or spraying and this makes the cost very high. In addition, these methods are incapable of providing a cladding on a surface that is difficult of access, as on the inner surface of a small diameter pipe.
On the other hand, it has been proposed to produce clad products using the well-known hot isostatic pressing method. For example, Japanese Patent Public Disclosure 61(1986)-223106 discloses a method for high efficiency production of alloy clad products by heating high alloy powder to a temperature above the solidus while subjecting it to gas pressing. However, in the disclosed method, as well as in all other methods employing hot isostatic pressing that have reported, the method of producing the clad product is carried out on a finished product and, as a result, the cost is high. Moreover, these methods are incapable of producing large products or long products measuring, for example, 12 meters or more in length.
Further, in Japanese Patent Public Disclosure Nos. 61(1986)-190007 and 61(1986)-190008 there are disclosed methods wherein a powder is charged into a capsule formed of a thick malleable metal cylinder and a thin metal cylinder of different diameter from the thick cylinder, the capsule is subjected to cold isostatic pressing to compress the powder into a billet, and the billet is subjected to hot extrusion, or wherein a double-walled vessel consisting of two concentric cylinders one inside the other is made of rubber or like material, a cylindrical malleable metal material is accommodated in the vessel in intimate contact with one of the vessel walls, powder material is charged in between the other vessel wall and the aforesaid cylindrical material and, after being sealed the vessel is subjected to cold isostatic pressing, the material thereafter removed from the vessel being used as a billet to be subjected to hot extrusion. However, these methods are unable to overcome the problem that when hot working is carried out on an assembly consisting of a metal substrate clad with a material exhibiting a large flow stress such as Hastelloy C-276, Inconel 625 or other nickel alloys or the like, the joint strength between the metal substrate and the cladding is weak so that the cladding is apt to separate from the metal substrate or suffer cracking.